# hv_vmx_vcpu_get_cap_write_vmcs(_:_:_:_:) Returns the allowed_0 and allowed_1 masks for a VMCS field of a vCPU. ``` func hv_vmx_vcpu_get_cap_write_vmcs(_ vcpu: hv_vcpuid_t, _ field: UInt32, _ allowed_0: UnsafeMutablePointer, _ allowed_1: UnsafeMutablePointer) -> hv_return_t ``` ## Parameters `vcpu` The vCPU ID. `field` The ID of the VMCS field for which to return capabilities. `allowed_0` The pointer to the VMCS allowed_0 mask (written on success). `allowed_1` The pointer to the VMCS allowed_1 mask (written on success). ## Return Value [`HV_SUCCESS`](/documentation/Hypervisor/HV_SUCCESS) if the operation was successful, otherwise an error code specified in [`hv_return_t`](/documentation/Hypervisor/hv_return_t). ## Discussion Returns the constraints imposed by the Hypervisor framework on the given VMCS field, in the form of `allowed_0` and `allowed_1` masks, that indicate what bit values may be set when writing the given VMCS field. When writing to a VMCS field, the caller is allowed to set bits that are 0 in its `allowed_0` mask to 0, and bits that are 1 in its `allowed_1` mask to 1. This means: - If allowed_0 = 0, allowed_1 = 0 -> must be `NOT SET`. - If allowed_0 = 0, allowed_1 = 1 -> can be either `SET` or `NOT SET.` - If allowed_0 = 1, allowed_1 = 0 -> undefined (shouldn’t happen). - If allowed_0 = 1, allowed_1 = 1 -> must be `SET`. > Important: > This function must be called by the owning thread. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)